ABSTRACT A fundamental goal in developmental biology is to understand how distinct cell types containing the same genetic information arise from a common precursor cell. Sex determination is a key process that results in a unidirectional commitment of initially bipotential cells towards either the male or female fate, making it a unique model to study cell fate commitment and differentiation in vivo. Despite several decades of research, it is still unclear how epigenetic regulation is integrated with transcription factor binding to induce gene activation and repression during cell fate decisions. We found that epigenetic mechanisms contribute to the bipotential state of the fetal gonad and to the regulation of chromatin accessibility during and immediately downstream of the primary sex-determining switch. We generated an epigenetic profile of the supporting cell lineage of the gonad before and after sex determination, and showed that a number of sex-determining genes are bivalent at the bipotential stage, marked with both active H3K4me3 and repressive H3K27me3 histone modifications. We found that activation of the male or female pathway is accompanied by a drastic loss of H3K27me3 at male- or female-determining genes, respectively. In contrast, repressed genes that promote the alternate pathway remain bivalent even into adult life. There are two H3K27me3-specific histone demethylases: Kdm6b, which is autosomal and expressed at similar levels in both XX and XY cells, and Utx (Kdm6a), which escapes X- inactivation and is expressed at higher levels in females than in males. Utx is one of few genes that escape inactivation in both mice and humans, suggesting an evolutionarily-conserved dose-dependent function. Although Utx has a homolog on the Y- chromosome (Uty), studies suggest that it is catalytically inactive and functions through H3K27me3-independent mechanisms. We will investigate the hypothesis that Utx and/or Kdm6b is involved in establishing male of female fate through analysis of mutant mice. Given the potential to link sex chromosomes to sex determination, and/or to demonstrate an evolutionary link to the recently discovered role of Kdm6b in reptile sex determination, there is a high likelihood that this work will result in an important publication in the field. It will also advance training of the post-doctoral fellow (Sara Alexandra Garcia-Moreno) in enhancer/promoter interactions and their integration with epigenetic marks, bioinformatics, immunofluorescent imaging, the analysis of mouse mutants, and other career skills in preparation for the next step in her trajectory toward becoming an independent research scientist.